Equalizer



United States Patent'fi This invention relates; to wave transmission. networksw and more particularly to adjustable equalizers;

An :object' of, the invention is, to, select a, desiredprow portional' part of the discrirr'iination characteristicf flf an equalizer andhat the, samentime adjust the, attenuation level as desired; A further objectisflto keep, the attenuae tion. substantially constant, at anarbitrarily chosen fre'-. quency as, the equalizerv characteristic is, adjustedl In, ,one embodimenththe, equalizerin accordance with; the present invention is a bridged-T structure compris:,- ing two. series resistors which are.ordinarily equal,' an interposed. shuntbranclii and a. bridging branch, The. bridging branch and. the; shunt, branch, have an iiri: pedance product which is substantiallyconstant at-nall. frequencies. of interestandeach comprises anQim edancer. a transforming, attenuator ,terminatedlin Ia fixed impedance. 7 Each attenuator includes means for simultaneously chang ing. both its. attenuation and its, ratio, of i impedance ,transformation.v As ,a special; case, the attenuation: of the-.11. equalizer may, be kept. substantially, constant,;.,atr an;- arbitrarily, selected. frequency, ,as: the equalizer l is ad-.'- justed to. obtain various proportional parts of a desired. discrimination characteristic.. Each" attenuators will; 3 ,in general, comprisevthree, resistorsgconnectedyiriaa {P011 1r '2 formation.

The nature of the invention will be more fully; under- 40 stood. from the 1 following. detailed. idescriptionr and; by referenceato the accompanying drawings; ofi whioh Fig. 1 isaschematic circuitoflanequalizer;inaocordea ance, with the present invention inthe form; of tant um; balanced bridged-T- structure;-- 1

Figs, 2,3, 4 and 5 ShOW- V3Ii0ll8ifOIII1Sv0f the-bridg ng". branch. of the equalizer to WhiChIGfCITCHGC'JSE made explaining; the theory ofthe inventiong v Eg 6, gives typical attenuation frequency 'charac'zteris-t ties at, three .settingsof an; adjustabletequalizer-known: to,the;prior art;- y Y Fig-, 7 gives,;-the corresponding. characteristics; of am;- adjustable equalizer in accordance with the inventiorm in;, which the, attenuation. at: the carbitraiilyn'selected :frequency is, remains unchangedeatallisettings;:ande1 Figh 8' shows in detail: theycirouit 0f" an adjustable 11 equalizer in ,accordancewitlnthe invention; in the form of; a balanced bridged-T structure;

Takingup the figures in more detail, Fig; 1 shows one embodiment of an equalizerrin:accordance:with'nthe:invention I in;;the form; of :an I unbalancedv bridged-T network having a pair;of;=-input:terminalsr1, '2, tOWhiCh a suitable-source of: alternating signalsmay be: connected; andrja pair of output :terminals 3, 4; to' which a load" ofgsuitable impedance may be sconnected. j The network is :az'constant-resistance structure of I the general type disclosed; in i United States Patent 1 No; 1,606,817, to 'G; Stevenson, issued November. 16,4926, and," 'asindieated, hassiateachaend a substantially nonereactiveimage impedanceRowhichis substantially constant with-frequency -q throughout l the operating range; It comprises-two equalg seriesrresistors each of value -Rb,- a bridgin'g liranch'*1'0-"' of impedance Z1 connected between-theouter 'terminals 5 and 6 thereof, andalshunt branch?ll rof impedance Z2 connected between the-,commonyterminalfi of the resistors and a point,8. in;thevpathgbetweenw terminals 2 and 4. The impedance- Z1;comprisesiatgfour-terminah impedance-transforming;attenuator-NL terminated in a fixed impedance X ,-,and ,-theimpedance Z2 comprises another impedanceetransfiorming"t attenuator N2 terminated in a second fiKfid MPQQAQGQjXL- As indicated by I 2,694,184 Patented Nov. 9, 19 54 2"; theearmws'econnectedz; by-x theizdotiand-dashglinea: 12; their networks 1n;and..N-2.:are':adjustable andnmayibe undem unitary control. TherimpedanaesxXAt-.and:XB,1-have :ap Y proximately;stheirelationshipp into; aibalanced"bridgedflfstructure offj'tlie type showiil' in' iFigr, '8"; if desired;

The" theory underlying; ,theinvention" will now be vex. plaihed' with; the; aid'bf Figs, ,2, 3174 and '5' showingvar" ous bridging branchesi whichf mayjbei .connected; betvveen1: points 5 "and6 fofiith'ej network of -Fig; ,1; Asisho'wngi combination :,of,' an .nnp'edance and? a; resistanceLRf. which, will, provide, a desired attenuation-frequency- Q char; acteristict'forthe equalizer, I

Now, a .fra'ctiont'or multiple); kffof Ltliisfcharac-teris'tio'. can be tobt'ainedlo a good. approxirnation1by substiti1tirig. for X and R'the impedance X"and the resistance R-",as'.:. shown, .in, Figt fi provided, .the 7 following relationships obta'in'ig pedanceXji and: the shunt resistance R byfan *ll-typee.

attenuator'xcomprising 'a'series resistance Rrand a shunt "resistance R 'z,, as shown.in Big. 4. An equalizer with,

a bridging branch asshown' inFig. 4 will have the same attenuation-frequency characteristic, except for. an added constantsloss 1ofj'A'i nepers, -:as; one with the branch shown inFigL; 3L iflth'e "following relationships :are. -satisfie'dz 2."si h($55)" sinh-v I In particular, if thepmagnitude" of A1 isfiXedbyLtHe-t relationship therr-the impedance )(r*/will-,be equal to X and thus any desired fraction, k, of the original characteristic of the equalizer employing the branch shown in Fig; 2 can be obtainedilKexcept:for-rhea added; constantalosszAiiyi with the? original fixed-impedance X.' Since any "desired pro portional :part; of: the"; original discrimination characteris ticilc-anxbe r realizedrsinzthise mannere it; is possiblev zto oh tamramad ustahle'l equalizervthi ough-ianiappropriate se'leea tion or adjustment of the resistances R1 and R: of Fig. 4, as indicated by the arrows, while having the impedance X1 fixed. Typical attenuation-frequency characteristics thus obtainable, with values of k equal to unity, /3 and V6, are shown by the curves 13, 14, 15 and 16, respectively, in Fig. 6. In this figure. and also in Fig. 7, a logarithmic frequency scale is used.

In many applications of adjustable equalizers it is desired that the loss of the equalizer at an arbitrarily selected frequency, such as fs in Fig. 7, remain fixed for all steps in the adjustment, a feature unobtainable with equalizers of the prior art. However, in accordance with the present invention this may be accomplished by an equalizer whose bridging branch 10 has the configuration shown in Fig. 5, comprising a network N1 terminated in an impedance Xx. The network N1 is a fourterminal, impedance-transforming attenuator in which both the ratio of impedance transformation and the attenuation may be selected or adjusted to have the required values. At least three impedance branches are required for the network, and these may be arranged in the form of either a T or a 1r structure. As shown, the network N1 is of the T-type, comprising two series resistances RA and R0, with a shunt resistance Rn interposed therebetween. In order to facilitate adjustment of the network, each of these resistances may be made adjustable, as indicated by the arrows, and may be placed i ndei7unitary control, as indicated by the dot-and-dash Typical attenuation-frequency characteristics thus obtainable are shown in Fig. 7, where the curves 19, 20, 21 and 22 correspond, respectively, to a k of unity,%, and As and each has the same attenuation at the arbitrarily chosen frequency fs of 0.25 megacycle. Since these characteristics are of the type having a maximum loss at a finite frequency, they will also cross at a second common frequency x, which is 4 megacycles in this case. To obtain these characteristics, a second loss of A2 nepers, constant with frequency but dependent on k, is added to each of the characteristics of Fig. 6. The impedances shown in Fig. 5 have the following values in terms of impedances already known:

RA=[A2]R1 RB= [e lR-z (11) Re: fe -11m) (12) X4: [EA2]X1 (13) The value of the impedance XA as thus determined is not constant, since it depends upon A2 which, in turn, varies with the value of k. However, it will be noted that, for equal losses A and A2, the impedance-multiplying factor in Equation 8 is much larger than the corre sponding factor in Equation 13. It follows, therefore, that by a readjustment of the relative proportions of A1 and A2 the desired result of an invariable impedance Xx with (a constant loss at a selected frequency can be obtaine 1 The formulas giving the required losses A1 and A: are quite involved, so that a method of successive approximations affords the easiest means for solving for their values. The following method is suggested. Let is be the frequency at which it is desired to maintain a fixed loss for all values of k, and let As be the loss at this frequency when k=1. Find (A1+kAS) as a function of k, using Equation 9 to determine A1. The quantity (Ai-j-kAs) will have a maximum for some value of k. If the maximum occurs when k==l, choose a loss A2 such that the sum S, given by A kA sinh s1nh If, on the other hand, the maximum value of (A1 kAs) should occur when lc l, this procedure would require a negative value of A1, to satisfy Equation 15 when k=1, and an alternative procedure must be followed. Choose a trial sum S slightly higher than the maximum value of (Ai-l-kAs). Find A: for k: 1, and compute 8 for this case. The object is to find values of A1 and A: such that the overall impedance-multiplying factor will satisfy the equation sinh A, kA SlHh(2 S1nh(2) where A21 is the value of A2 for k=1.

Examining first the value of k which gives the maximum (At-j-kAs), it will be found that too low a choice of S will require a negative value of A2 to satisfy Equation 16. Too high a choice of S will result in more added loss in the equalizer than necessary. The optimum choice is that which makes A2=0 for the value of k corresponding to maximum (A1+kAs). After the appropriate value of S has been determined, Equation 16 can be satisfied for each value of k by read usting the values of A1 and A: by successive approximations, keeping S fixed.

Finally, the element values may be determined using Equations 10 to 13 inclusive. In this manner the resistance values are obtained at discrete steps, k, in the ad justment. If the adjustment is to be continuous, intermediate values can be obtained by plotting and inter polation.

1n the theory outlined, it is assumed that the fixed impedance branches comprise pure reactance networks. Although this represents the most important case in practical applications, the fixed impedance branches may comprise generalized irnpedances provided any equiv alent parallel resistance in the branch is associated with network N1.

The values of all the elements of the bridging branch shown in Fig. 5 have now been determined. This branch may be used for the corresponding one shown in Fig. 1. The impedance of the branch XB is found from Equation 1. The network N2 is determined by the relationship given in Equation 2. The network N1 may, if desired, be transformed into an equivalent 1r network. Likewise, the network N2 may be either of the T type or the 1r type.

Fig. 8 is a schematic circuit of an equalizer in accordance with the invention in the form of a balanced bridged-T structure which will rovide the characteristics shown in Fig. 7. Each of e networks N11 in the bridging branches is the same as the network N1 of Fig. 5, except that each of the resistances in N11 has only half the value of the corresponding one in N1. Each of the series resistances has a value of Ro/ 2. Each of the networks N11 is terminated in an impedance XA/Z comprising three parallel arms. One of these arms is an inductance Ll, another is a capacitance C1, and the third is made up of inductance L2 and a capacitance Ca in series. 7

The shunt impedance branch Z2 comprises a network N2 terminated in an impedance X13. The network N2 is a T-type structure comprising two series resistances Rx and Rz and an interposed shunt resistance RY. The impedance Xn comprises the parallel combination of an inductance L4 and a capacitance C4, in series with an inductance L3 and a capacitance C3.

All of the elements in the branches XA/ 2 and XB may be fixed in value. However, provision is made for changing both the ratio of impedance transformation and the attenuation of the networks N11 and N2. This may be done by substituting resistances of the required value or, as indicated by the arrows and the connecting dot-and-dash lines, the component resistances may be made adjustable under unitary control.

To provide the characteristics shown in Fig. 7, the reactive elements of Fig. 8 will have approximately the following values when the image impedance R0 is ohms, the frequency is of invariable loss is 0.25 megaeycle, and A0 is 0.4385 neper:

Ci=577 micro-microfarads C2=675 micro-microfarads Ca: 1 338 micro-microfarads C4=293 micro-microfarads L1=8.l0 microhenries La=1.773 microhenries L3=3. 492 microhenries 14=4.09 microhenries Each of the resistances Ro/ 2 has a value approximately equal to 55 ohms.

For the curve 20 of Fig. 7, corresponding to k=-%, A1 may be taken as 0.0395 neper and A2 as 0.0213 neper. The component resistances in the networks N11 will then have approximately the following values:

RA/2=1.18 ohms RB/2=28.0 ohms Rc/2=2.'46 ohms For the curves corresponding to other values of k, these resistances will have other values, determined as explained above, such that the loss of the equalizer at the frequencies fs and fx always remains the same.

What is claimed is:

1. An equalizer of the bridged-T type comprising two series resistors, an interposed shunt branch, and a bridging branch, said branches having an impedance product which is substantially constant at all frequencies and each comprising a four-terminal, impedance-transforming attenuator and a fixed reactive impedance connected to the output terminals of said attenuator, the input terminals of one of said attenuators being connected to the outer terminals of said resistors, an input terminal of the other of said attenuators being connected to the common terminal of said resistors, and each of said attenuators comprising three unequal resistances connected to form a ladder-type network and including means for changing the value of each of said three resistances, whereby the attenuation of the equalizer may be kept substantially constant, at an arbitrarily selected frequency, when said resistances in said attenuators are changed to obtain different proportional parts of the discrimination characteristic of the equalizer.

2. An equalizer in accordance With claim 1 in which said two series resistors are approximately equal in value.

3. An equalizer in accordance with claim 1 in which said three resistances in each of said attenuators are connected to form a T network.

4. An equalizer of the bridged-T type comprising two approximately equal resistances connected in series, a shunt branch interposed between said resistances, and a bridging branch connected between the outer terminals of said resistances, the product of the impedances of said branches being substantially constant throughout the frequency range of interest, each of said branches comprising a four-terminal, impedance-transforming attenuator and a fixed reactive impedance connected to the output terminals of said attenuator, the input terminals of one of said attenuators being connected to said outer terminals of said resistances, an input terminal of the other of said attenuators being connected to the common terminal of said resistances, and each of said attenuators comprising three unequal resistances connected to form a ladder-type network and including means for changing the values of said three resistances to other values so chosen as to provide a proportional part of the original discrimination characteristic of the equalizer throughout said range without changing the attenuation at an arbitrarily selected frequency.

5. An equalizer in accordance with claim 4 in which said three resistances in each of said attenuators are connected to form a T network.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,606,817 Stevenson Nov. 16, 1926 2,153,743 Darlington Apr. 11, 1939 2,238,023 Klipsch Apr. 8, 1941 2,374,872 Lundry May 1, 1945 

